Assembled multi-surface optical component and method for fabricating

ABSTRACT

An assembled optical component has a support structure with a reference surface at which a number of individual optical elements are bonded at predetermined positions. The curvature of the reference surface is selected such that optical surfaces of the optical elements are in a predetermined orientation at their assembly positions. The optical elements are preferably planar mirrors simultaneously fabricated from a wafer. An adhesive film attached to the wafer prior to separation of the optical elements assists in temporarily positioning the elements on a temporary fixture, which holds the elements in position, while they are bonded to the support structure.

CROSS REFERENCE

This application cross-references the U.S. patent application titled“Optical Cross-Connect Switch with Telecentric Lens and Multi-SurfaceOptical Element” filed by inventors Dmitry V. Bakin and Cheng-ChungHuang on Jan. 29, 2003, U.S. patent application Ser. No. 10/354,887,which is hereby incorporated by reference.

FIELD OF INVENTION

The present invention relates to optical components having multipledistinct optical surfaces and a method for fabricating it. Moreparticularly, the present invention relates to a multi-surface reflectorof an optical crossbar switch and a method for fabrication thereof.

BACKGROUND OF INVENTION

With the advancement of optical telecommunication technologies opticalcomponents become increasingly complex and sophisticated in design. Inan optical crossbar switch, also known as an optical cross connect(OXC), a multitude of optical communication lines may be simultaneouslyswitched. The switching is typically performed by spatially directingfocused signal beams between optical fiber interfaces. The focusing of asignal beam is commonly accomplished by placing a lens in front of thefiber end. This means that in a fiber interface with two dimensionallyarrayed fiber ends lenses are arrayed in axial alignment with each fiberend.

Fiber interfaces are fabricated with ever increasing numbers of fiberswhile reducing the pitch between individual fiber axes. As aconsequence, the fabrication of Lens arrays becomes increasinglychallenging and cost intensive. To circumvent this problem, a modifiedOXC may be configured with a telecentric lens that simultaneouslyfocuses a number of signal beams propagating towards and away from thefiber ends. In that context it is referred to the cross-referencedapplication for “Optical cross connect with simultaneous focusing ofdiscrete signal beams”.

A core component of such a modified OXC is a multi-surface opticalcomponent that is placed after the telecentric lens. The multi-surfacecomponent has a number of individually positioned optical surfacesconfigured and positioned such that each of the simultaneously focusedsignal beams impinges on a predetermined optical surface and is directedonto a moveable mirror element within a mirror array where the signalbeams are spatially redirected for switching purposes.

In the preferred embodiment, the optical surfaces are planar mirrorsthat direct the signal beams onto individual mirrors within the moveablemirror array by means of reflection. The efficiency and dimensionalscale of the modified OXC is highly dependent on the position andorientation precision with which the individual mirrors are positionedand oriented on the multi-surface component.

Optical components with multiple optical surfaces have been fabricatedin several ways. In the case where a relatively low number of opticalsurfaces are combined and spatially arrayed with an angle betweenadjacent optical surfaces of more than 180 degrees, the fabrication isrelatively easily accomplished. For example, U.S. Pat. No. 5,692,287 toNakamura et al teaches a method for making a polygon mirror by machiningthe mirror surfaces from a monolithic metal block. As can be seen in theFigures, the fabrication of the mirror surfaces is relatively simplesince the machining tool may extend beyond the individual mirror'sboundaries without interfering with other mirror surfaces. Also thenumber and arrangement of the individual mirror surfaces does not imposeunusual effort in the setup process of the work piece on the fabricationmachine.

In cases where a high number of small optical surfaces needs to befabricated with high precision into a single optical component,machining of the individual optical surfaces becomes arduous. For eachoptical surface, the monolithic block would need to be positionedaccurately with respect to the machining tools machining plane. In caseswhere the optical surfaces are spatially positioned relative to eachother, accurate machining positioning is difficult to accomplish.Secondly, the machining of a high number of independent optical surfacesinto a single work piece bears an increasing risk of machining errorsthat grows with the number of optical surfaces.

In cases where the angle between adjacent optical surfaces is less than180 degrees, machining becomes much more complicated, since themachining tool may not extend beyond the intersections of adjacentoptical surfaces. Hence, machining is typically a highly unfeasiblefabrication method for optical components with concavely arrayed opticalsurfaces.

In a modified OXC, the multi-surface component has to provide a numberof discrete optical surfaces that is at least as high as the number ofswitched lines. As the switching capacity of an OXC advances tosimultaneous switching of several thousand signal beams, there arises aneed for new ways of efficiently fabricating a multi-surface component.

In one approach, individual optical elements are prefabricated with asingle discrete optical surface. The optical elements are then assembledtogether in a one by one fashion. This is accomplished by spatiallypositioning each optical element in a fixture while bonding them to oneanother or to a support structure. The fixture provides the accuratepositioning of the optical element while the bonding takes place. Thespatial fixing of the optical elements requires the separate adjustmentof six degrees of freedom (Translations in X,Y,Z and Tip, Tilt, andClocking) for each individual mirror element. Even though this methodmay have some use in cases where a low number of optical elements arecombined in a single optical component, the method is highly unpracticalfor fabricating optical components having a large number of discreteoptical surfaces.

Therefore, there exists a need for an efficient and precise fabricationmethod for optical components having a large number of optical surfaces.The invention described in the following addresses this need.

SUMMARY

In an optical cross bar switch also called an optical cross connect(OXC), a multi-surface optical component has a number of opticalsurfaces that are spatially arrayed and positioned in a predeterminedfashion. For the purpose of ease of understanding it is referred to theschematic FIG. 1. There, a simplified multi-surface component 1 hasoptical surfaces 10 that may be defined by the spatial position of theircenter points 11 and the spatial orientation of their center axes 12.

In the preferred embodiment of the invention, a fabrication method isdisclosed for a multi-surface optical component 1 with center points 11and center axes 12 being geometrically correlated to a continuousgeometrical surface 2. In the example of FIG. 1, where all center axes12 intersect in a common point 4 and the center points 11 have equaldistances to the point 4, positions and orientations of each mirror 10may be modeled in a fashion similar to that of a well-known sphericalreference surface 2.

As may be well appreciated by anyone skilled in the art, the referencesurface 2 may be elliptical, hyperbolical, parabolic, aspheric or mayhave any other continuous geometrical surface. Moreover, the referencesurface 2 may in fact be an offset surface 6 from the center points 11.Each mirror's 10 position and orientation may still be modeled by simplyincluding the offset distance 5. The present invention takes advantageof this fact and provides a support structure with a reference surfacesuch that optical elements each having an optical surface are referencedand fixed in a predetermined spatial position and orientation. A novelmethod is introduced to simultaneously position all optical elementsprior to bonding them on the reference surface. As a result, amulti-surface optical component may be fabricated in an efficientfashion substantially independent of the number of optical surfaces ofthe optical component.

Each of the independent optical elements has on one side an opticalsurface and on the opposite side a reference feature with which theoptical element is brought into contact with a predetermined area of thereference surface whereby position and orientation of the opticalelement and consequently its optical surface is defined. In otherembodiments, the position and orientation of the optical elements aredetermined by the reference surface contacting the optical surface ofthe independent optical elements. The optical elements are rigidly heldin place while being bonded to the reference surface.

In the preferred embodiment, the optical elements of an assembledmulti-surface optical component are simultaneously fabricated from awafer. In an initial fabrication step, an initial optical surface isfabricated on top of the wafer. In the preferred case where the opticalelements' optical surfaces operate as mirrors, the wafer top is simplycoated with well-known layer(s) that provide the desired reflectivity.Then, an adhesive film is attached to the wafer top and the referencefeatures of the individual elements are shaped on the bottom side of thewafer. When the optical elements are separated via the wafer's bottomsurface, the adhesive film holds on to the separated optical elements.This is accomplished by maintaining the structural integrity of theadhesive film during the separation of the optical elements.

The definition of the reference features and the separation of theoptical elements may be preferably accomplished by simply cutting thewafer in a predetermined pattern. The corners formed between angulatedcutting gaps and the remainder of the wafer's back surface define thereference features. This simple way of separating the optical elementswhile creating the reference features is applicable in the case of aconcave reference surface. The concave curvature of the referencesurface provides clearance to the back optical elements' back surfaceswhile in contact with the elements' corners.

In the case of a convex curvature of the final reference surface, thereference features may be fabricated into the wafer's bottom surfaceindependently to the step of separating the individual optical elements.Cavities may be formed by well-known etching operations at locations ofthe wafer bottom surface that correspond to the central bottom areas ofthe separated optical elements. The reference features may be definedthereby as edges or corners between cavity walls and the remainder ofthe wafer's bottom surface.

Once the optical elements have been separated and while the elementsadhere to the adhesive film, the film is stretched over a temporaryreference surface of a temporary fixture such that the referencefeatures point away from the fixture. The curvature of the temporaryreference surface is in an approximate offset to the final referencesurface such that all reference features snuggly contact the finalreference surface of the support structure once the reference featuresare forced against the support structure via the temporary fixture.

In the simplest case, the cutting operation provides not only fordefining the shape of each optical element and reference features, butalso contributes with its cutting gap in correspondence with the filmsstretch characteristic and an eventual stretching procedure to the finalassembled position of the optical elements. The eventual stretchingprocedure may include a variation of a stretching force duringstretching over the temporary fixture and/or by selectively establishingan adhesive connection between the film and the temporary referencesurface. The film also may be stretched over the temporary fixture by afluidal pressurization of it. Fluidal pressurization may be particularlyused where the temporary reference surface has a concave curvature incombination with a convex curvature of the final reference surface.

Once the reference features are brought into contact with the finalreference surface, the optical elements are bonded to the supportstructure by use of a fixing medium. The fixing medium may be a curablegel, epoxy, casting compound, or other adhesive or a solder, weld, orother metallic joint. The adhesive may be cured by UV radiation througha translucent support structure. To avoid eventual outgasing of thefixing medium and/or other affiliated components and to hermeticallyseal the gaps between the optical elements, the optical elements may besoldered or brazed to the support structure instead. In that case, thesupport structure may be a glass material that resists the soldering orwelding temperatures. The reference surface is coated with a first metallayer that adheres to glass and a second metal layer that adheres to thesolder and to the first metal layer. To minimize thermal deformationduring the soldering process, the support structure may be made of fusedsilica.

The scope of the invention includes embodiments, in which the opticalsurfaces are non planar. In addition, by selecting the optical elements,the support structure and the fixing medium from materials havingsubstantially the same optical properties a translucent multi-surfacelens may be fabricated as well.

Further, the scope of the invention includes embodiments, in which theoptical component is a multi-surface lens. In that case, the fixingmedium, optical elements and support structure are translucent and havesubstantially the same optical properties, such that the beams impingingon the optical surfaces may propagate through the optical componentsubstantially unaffected by interfaces between the fixing medium, theoptical elements and the support structure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a simplified section view of an optical component and itsgeometrically defining elements.

FIG. 2 depicts a fabrication step of making an optical layer on a wafer.

FIG. 3 depicts a fabrication step attaching an adhesive film to thewafer of FIG. 1.

FIG. 4 depicts a fabrication step of cutting the wafer of FIG. 1 intooptical elements.

FIG. 5 depicts a fabrication step of stretching the adhesive film withthe optical elements of FIG. 4 over a temporary fixture.

FIG. 6 depicts a fabrication step of positioning the temporary fixtureof FIG. 5 and contacting the optical elements with a support structure.

FIG. 7 illustrates a final assembled optical component in accordancewith the fabrication steps depicted in FIGS. 2-6.

FIG. 8 shows a detail view of a single optical element fixed on thetemporary fixture via the adhesive film.

FIG. 9 shows a detail view of the optical element of FIG. 8 with itsreference features contacting the support structure.

FIG. 10 shows a detail view of the optical element of FIG. 9 bonded tothe support structure in the position depicted in FIG. 9.

DETAILED DESCRIPTION

Referring to FIG. 7, an assembled multi-surface optical component 1includes a support structure 500 having a reference surface 501 and anumber of separate optical elements 110 that are bonded to the supportstructure 500. Each optical element 110 has an optical surface 10 thatis spatially arrayed and positioned in a predetermined fashion. As canbe seen in the detailed view of FIG. 10, the distinct orientation of theoptical element 110 is provided by contact of its reference features 106with the reference surface 501.

Now referring back to FIG. 1 the geometric relationship between spatialposition and orientation of the optical surfaces 10 on one hand and thereference surface 2, 6 on the other hand is described in detail. Opticalsurfaces 10 may be defined by the spatial position of their centerpoints 11, the spatial orientation of their center axes 12 and theirpitches 13, (see also FIGS. 5, 7). Pitches 13, are shown in a simplifiedfashion. It is clear to anyone skilled in the art that pitches may varyfor accomplishing various assembly patterns of optical surfaces withinan assembled multi-surface component 1. Further it is noted that thescope of the preferred embodiment includes cases where the surfaces 10do not contact directly but may be separated by a gap or a shoulder orany other structural configuration appreciated by anybody skilled in theart for rigidly holding the individual elements 110 (see FIGS. 4-10).

According to the preferred embodiment of the invention and the teachingspresented in the following together with FIGS. 2-7, a fabrication methodis disclosed for a multi-surface optical component 1 with center points11 and center axes 12 being preferably geometrically correlated to acontinuous geometrical surface 2 and 6, in which surface 6 is an offsetsurface of surface 2. In the example of FIG. 1, where all center axes 12intersect in a common point 4 and the center points 11 have equaldistances to the point 4, positions and orientations of each surface 10may be modeled in a fashion similar to that of a well-known sphericalgeometry of reference surfaces 2 and 6.

The reference surfaces 2 and 6 may also be elliptical, hyperbolic,parabolic, aspheric or any other well-known continuous geometricalsurface. It is clear that the teachings presented in the above withrespect to point 4 and radius 7 are applicable only for sphericalreference surfaces 2 and 6. Nevertheless, position and orientation ofeach surface 10 may be modeled as a function of the reference surface's2 defining parameters and the pitch 13. (Please note: labels 13A and 13Bdo not appear in any figures.) These findings are utilized by providingthe final reference surface 501 corresponding to surface 6 and atemporary reference surface 401 (see FIGS. 5, 6, 8 and 9) correspondingto surface 2. The temporary reference surface 401 has a distinctfunction during an inventive fabrication method of the optical component1, which will be described in the following.

Referring to FIG. 2 and a preferred embodiment, the optical elements 110are made of a wafer 100. In the case where the optical surfaces 10 areplanar mirrors, a reflective coating 102 such as gold may be depositedon the wafer top surface. The invention includes embodiments, wherereflectivity is established on the top surface by other well-known wayssuch as polishing and/or deposition of other feasible materials. Hence,the fabrication of the optical surfaces 10 is accomplished in asimultaneous fashion and highly independent of the number of separateoptical surfaces 10 of the final optical component 1.

The wafer 100 has also a bottom surface 101, which may be accessedduring further fabrication steps after the step of attaching an adhesivefilm 201 to the wafer's initial optical surface 102 fabricated on itstop. This is illustrated in FIG. 3.

As shown in FIG. 4, the wafer bottom 101 is utilized for referencefeatures 106, by forming corners and/or edges between cavities 103 andthe remaining bottom surface 101. Due to the highly precise thicknesscommon for wafers, the reference features 106 are fabricated with a highdegree of parallelism.

In the simplest case, the reference features 106 are defined between thebottom surface 101 and cutting gaps resulting from separating the wafer100 into individual optical elements 110. The separation is accomplishedwhile maintaining the structural integrity of the film 201. Separationmay be by sawing or Deep Reactive Ion Etching (DRIE) or other materialremoval process. The simplest case is applied where the final referencesurface 501 is concave.

During the separation not only the size of the optical elements 110 butalso their initial pitch 3 is defined. Consequently, any shape and arrayconfiguration of the optical elements 110 may be defined in a simplefashion and also highly independent of the number of optical elements110 involved.

After the optical elements 110 have been separated, the adhesive film201 holds them together while maintaining the initial pitch 3. In a nextstep, depicted in FIG. 5, the adhesive film 201 is stretched over thetemporary reference surface 401 of a temporary fixture 400. While thecurvature of the film 201 is brought from planar to a curvaturecorresponding to that of the reference surface 401, the initial pitch 3is converted into the final pitch 13 with which the optical elements 110will be bonded onto the support structure 500 after the optical elements110 are forced with their reference features 106 into contact with thefinal reference surface 501 (see FIG. 6).

Precise spatial positioning of the optical elements 110 is accomplishedduring the step of stretching the film 201 over the temporary referencesurface 401, while the precise spatial orientation of the opticalelements 110 remains undefined. As can be seen in FIG. 8, this is mainlyrelated to the fact that the adhesive film 201 has to compensate for thedimensional discrepancy in the offset between the curved surface 401 andthe planar surfaces 10. The thickness of the adhesive film 201 is likelyto be insufficient to compensate for the discrepancy in the offset suchthat the optical surface 10 may eventually partially lift off from thecurved film 201. Only after the reference features 106 are brought intocontact with the final reference surface 501, does the precise spatialorientation become defined for each optical element 110 despite anambiguous contacting condition remaining between the film 201 and theoptical surface 10 (see FIG. 9).

The optical elements 110 are bonded to the support structure while theyare rigidly held via the fixture 400. In one embodiment, an adhesive maybe applied in the gap between the support structure 500 and the opticalelements 110. One way of doing this is by applying the adhesive ontoeither or both of the final reference surface 501 or the backsides ofthe optical elements 110 prior to forcing the optical elements 110against the final reference surface 501. Eventual excessive adhesive islaterally squished out of the interfacing volume between opticalelements 110 and the final reference surface 501. The separation gaps103 between the optical elements 110 assist thereby in directing theadhesive flow towards the assembly's circumference. Once the adhesive isapplied properly it may be cured. In case of a translucent supportstructure 500 and/or translucent optical elements 110 the adhesive maybe a UV curing gel cured by applying a curing UV light via the supportstructure 500 and/or the optical elements 110.

To prevent eventual outgasing of the fixing medium 600, the opticalelements 110 may alternatively be soldered and/or brazed to the supportstructure. In such a case, layer(s) may be deposited on the supportstructure and/or the optical elements to assist in establishing areliable mechanical connection between the optical elements and thesupport structure 500.

Since the reference surfaces 401 and 501 are continuous geometricsurfaces similar to those used for optical lenses and mirrors, thesupport structure 500 and/or the temporary fixture 400 may be providedby conventional lenses and/or mirrors. This additionally dramaticallyreduces fabrication efforts, since such continuous lens or mirrorsurfaces are relatively simple to fabricate.

Having the support structure 500 in a configuration similar to that of alens additionally assists in the case of bonding the optical elements110 to the support structure 500 by use of a UV curing adhesive. Thetranslucent characteristic of the support structure 500 thereby providesa uniform optical path for a reliable curing of the optical adhesive.

In the case of soldering or brazing the optical elements 110 to a lenslike support structure 500 made of glass, a chromium layer may beinitially deposited on the reference surface 501. Chromium is well-knownfor its advantageous adherence to glass. A gold layer may be depositedon top of the chromium layer to assure reliable mechanical connection tothe solder. Finally, a thin solder layer is deposited on top of the goldlayer prior to contacting the optical elements 110 with the referencesurface 501. In a following heating process, the solder is temporarilyliquefied and the optical elements 110 are soldered to the referencesurface 501. To reduce thermal deformation during the soldering, thesupport structure 500 may be made of fused silica.

The present invention includes embodiments in which the optical surface10 is adjacent to the reference surface 501. In that case, the referencefeatures 106 are placed together with the optical surfaces 10 on thesame side of the optical elements 110.

Accordingly, the scope of the invention described in the specificationabove is set forth by the following claims and their legal equivalent.

1. An optical component including multiple optical surfaces, saidoptical component comprising: a support structure including a referencesurface; a number of optical elements each of them including: an opticalsurface; and a reference feature placed in a predetermined fashion onsaid optical element; and a fixing medium for rigidly holding saidoptical elements on said support structure, wherein said opticalelements are arrayed on top of said reference surface by contacting saidreference surface with said reference feature, wherein each of saidoptical surfaces is in a distinct spatial orientation relative to saidsupport structure.
 2. The optical component of claim 1, wherein saidreference feature is placed on a rear surface of said optical element,said rear surface being on the opposite side of said optical surface. 3.The optical component of claim 1, wherein said reference feature isplaced together with said optical surface on the same side of saidoptical element.
 4. The optical component of claim 1, wherein saidoptical elements are simultaneously fabricated from a wafer.
 5. Theoptical component of claim 1, wherein said reference feature is providedby an edge formed by a cutout contour of said optical elements.
 6. Theoptical component of claim 5, wherein said reference feature is providedby corner points of said edge.
 7. The optical component of claim 1,wherein said support structure is a prefabricated lens.
 8. The opticalcomponent of claim 1, wherein said support structure is a prefabricatedmirror.
 9. The optical component of claim 1, wherein said opticalsurface is a mirror surface.
 10. The optical component of claim 1,wherein said optical surface is a lens surface.
 11. The opticalcomponent of claim 1, wherein said fixing medium is a UV curableadhesive.
 12. The optical component of claim 1, wherein said fixingmedium is a solder for soldering said optical elements.
 13. The opticalcomponent of claim 12, wherein said support structure is made of glassand wherein said reference surface comprises: a first metal layerdeposited on said glass and selected from a first group of metalsadhering to said glass; and a second metal layer deposited on said firstmetal layer, said second metal layer being selected from a second groupof metals adhering to said first metal layer and adhering to saidsolder.
 14. The optical component of claim 13, wherein said supportstructure is made of fused silica.
 15. The optical component of claim 1,wherein said fixing medium is a brazing material for brazing saidoptical elements.
 16. The optical component of claim 15, wherein saidsupport structure is made of glass and wherein said reference surfacecomprises: a first metal layer deposited on said glass and selected froma first group of metals adhering to said glass; and a second metal layerdeposited on said first metal layer, said second metal layer beingselected from a second group of metals adhering to said first metallayer and adhering to said brazing material.
 17. The optical componentof claim 16, wherein said support structure is made of fused silica. 18.A method for fabricating an optical component including multiple opticalsurfaces, said method comprising: fabricating an initial optical surfaceon the top surface of a wafer; attaching said wafer with said initialoptical surface on a first side of an adhesive film, wherein a bottomsurface of said wafer remains freely accessible; defining referencefeatures on said bottom surface; separating said wafer via said bottomsurface into a number of optical elements in correspondence with saidreference features while maintaining a structural integrity of saidadhesive film; contacting a second side of said adhesive film with atemporary reference surface of a temporary fixture, wherein saidreference features point away from said temporary fixture; positioning afinal support structure with respect to said temporary fixture, whereinsaid reference features contact a final reference surface of said finalsupport structure; bonding said optical elements to said final supportstructure; and removing said temporary fixture and said adhesive film.19. The method of claim 18, wherein defining reference features on saidbottom surface is an integral part of separating said wafer in as muchas said reference features are defined by edges formed between a cuttinggap and the remainder of said bottom surface, said cutting gap resultingfrom said separating.
 20. The method of claim 18, wherein contacting thesecond side of said adhesive film with the temporary reference surfaceincludes stretching of said adhesive film.
 21. The method of claim 20,wherein said stretching is dynamically adjusted as a function ofcontacting progression between said adhesive film and said temporaryreference surface.
 22. The method of claim 18, further comprisingestablishing an adhesive connection between said adhesive film and saidtemporary reference surface, wherein establishing the adhesiveconnection being performed concurrently with contacting the second sideof said adhesive film with the temporary reference surface.
 23. Themethod of claim 22, wherein said adhesive connection is regionallyestablishing in conjunction with a stretching characteristic of saidadhesive film.
 24. The method of claim 22, wherein said adhesiveconnection is regionally establishing in conjunction with a curvature ofsaid temporary reference surface.
 25. The method of claim 18, whereinsaid reference features are adhesively fixed on said final referencesurface during the positioning of the final support structure withrespect to said temporary fixture.
 26. The method of claim 18, whereinbonding said optical elements to said final support structure isaccomplished by filling a cavity between said adhesive film, saidoptical elements and said final reference surface with an adhesivefollowed by curing said adhesive.
 27. The method of claim 26, whereinsaid adhesive is a UV curable adhesive.
 28. The method of claim 27,wherein said final support structure is translucent and wherein a curinglight is applied to said adhesive via said final support structure. 29.The method of claim 18, wherein bonding said optical elements to saidfinal support structure is accomplished by soldering said opticalelements to said final reference surface.